6-O-Methylcellulose (6MC) and 2,3-di-O-methylcellulose (23MC), having every structural unit of them regioselectivery substituted, and ununiformly 2,3-di-O-methylated cellulose (23MCU), having a trace amount of unsubstituted glucose units (ii) in the same molecular chain, were used to investigate the structure of the scission point in enzymatic degradation with Trichoderma viride cellulase. For 6MC the glycosidic bond between two adjacent substituted units could be cleaved to give oligomers with a degree of polymerization of ca 8, while 23MC was not degraded. The characteristics of hydrolysis changed by substituted positions of substituents even though the same methyl groups were used. In addition, 23MC inhibited the hydrolysis of (carboxymethyl)cellulose. Interestingly, even in such unfavorable circumstances, 23MCU which was almost composed of 23MC was hydrolyzed, and further the initial velocity of this reaction was significantly high. Moreover, the amount of reducing ends produced corresponded to the amount of U, suggesting that only the linkages between U and 2,3-di-O-methylated units were cleaved because the cellulase could not cleave glycosidic bonds between two adjacent substituted 2,3-di-O-methylated units. By analyzing the kinetics of the cellulase reactions on the above cellulose model compounds, we were able to estimate the effect of the substituted regions on the hydrolysis in the cellulose derivatives. Especially, 2,3-di-O-methylated regions competitively inhibited the hydrolysis of U in other molecular chains. However, they activated the scission of U in the same molecular chain.